This invention relates to a processing method of making injection molded ferrous component parts having excellent soft magnetic properties, by a two stage sintering processing. The invented processing method is particularly suitable for making small component parts for miniaturized electrical and electronic equipment.
Injection molding of ferrous powders is a known art. For components such as yokes, for example, parts having soft magnetic properties are required. Such parts are known to be made from pure iron particles of 50-100 .mu.m average diameter, which are pressed at 5-7 tons/cm.sup.2 to produce green compacts. The green compacts are sintered in a hydrogen or dissociated ammonia atmosphere at temperatures of 1100.degree.-1400.degree. C.
The recent trends toward energy efficiency and light weight have prompted miniaturization of components and equipment in industries, and the demands for small thin-walled structural components have increased. But the conventional processing method was able to achieve only about 85-95% theoretical density, and the resulting parts made by the process lacked the proper soft magnetic properties, in particular, the residual magnetic flux density was low and such parts were unable to satisfy the product requirements for soft magnetic applications.
The present invention resulted from extensive studies by the inventors to solve such problems of low residual flux density and other problems associated with the conventional processing methods. The inventors found that a new processing method which combined selected steps of the conventional method with the newly discovered steps produced the best results.
For example, carbonyl iron powders of less than 15 .mu.m average particle diameter are mixed with 8-15 weight % of a thermoplastic resin binder. The mixture is kneaded in a kneader at 135.degree. C. for three hours, and the mixture is then cooled to room temperature. This step is followed by a further granulation step in the kneader to obtain a feed particles mix containing particles of about 1-5 mm diameters.
The feed particles at temperatures of 130.degree.-140.degree. C. are injection molded into molded parts at an injection pressure of 500-700 kgf/cm.sup.2 and at an injection cycle time of 20-40 seconds.
The molded parts are subjected to dewaxing, i.e. presintering, to reduce the carbon content in the molded parts, produced by the decomposing binder, to less than 0.05 weight % by heating the molded parts at relatively high temperatures in the range of 680.degree.-750.degree. C. The parts are further sintered at relatively low temperatures in the range of 870.degree.-910.degree. C., to obtain molded parts having density values of over 96% theoretical density.
The above process is followed by a further heating step of conversion sintering treatment, involving heating the sintered parts at 1050.degree.-1350.degree. C. in a reducing atmosphere or in a vacuum to produce molded parts having excellent soft magnetic properties, in particular, high residual magnetic flux density required for component parts.